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Quantum comms can be made even more secure

Digital signatures using quantum states

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Quantum mechanics can be used to create the “perfect” digital signature, but in practice, it's held back by the technical impossibility of retaining quantum states for more than a few seconds. Now, a group of researchers from the UK, Croatia and Greece is proposing a scheme they say would make quantum digital signatures (QDS) not just feasible, but usable.

The Achilles heel of QDS is that it depends on persistent quantum memory, they say, and this means real-world implementations wouldn't happen for years.

In a paper published in Physical Review Letters and also available at Arxiv, the researchers propose a scheme that allows the distribution of quantum digital signatures, but eliminates the need for quantum memory.

Instead, they say, their QDS works with today's linear optics.

“In a generic QDS protocol, the sender sends pairs of quantum states, quantum signatures, to the multiple recipients. The recipients store the signatures in quantum memory until the sender decides to send a particular message,” they write.

Using quantum states as the signatures ensures authenticity and nonrepudiability of the messages, but since a message might need to be verified years after its creation, current QDS schemes assume there's some way to store the signatures indefinitely – which there isn't.

“This is a serious shortcoming given that state-of-the-art quantum memories cannot achieve coherence times longer than minutes,” they write.

What the researchers propose is that the quantum states representing the signature information be converted to classical information using quantum measurements – and what the users store is the classical information representing the signature.

The researchers also note that the kinds of communication systems already used in commercial quantum key distribution (QKD) schemes could represent a practical way to implement their QDS protocol for experiments.

The paper was authored by Vedran Dunjko, Petros Wallden and Erika Andersson. ®